Bright, who plays Linda Carter in the BBC soap, revealed that lockdown had been the deciding factor in going for a third baby after sons Freddy, nine, and Gene, four.
She told OK! magazine that she had still had three frozen embryos from IVF treatment to conceive Gene, and then had two failed attempts at implanting one before becoming pregnant with the final embryo.
Bright said: "I had such an emotional attachment to those embryos. I know they’re cells but they have the potential to be another Gene! I couldn’t let it go. Eventually, we decided to go ahead at the end of 2019... and then COVID-19 happened.
"I felt like the rug had been pulled out from under me.
"Suddenly, [husband] Paul and I were having conversations about not having more kids. Everything felt so uncertain. But by the end of the first lockdown, we did a complete 180.
"Having that precious time at home with our boys, doing simple things, we knew we had to try to make it happen."
The actor said she and husband Paul Stocker had spent a couple of years trying to decide whether to have a third child, and that they had decided against a small gap between Gene and a new baby because of her busy soap role.
Bright said it was an emotional moment when she got a positive pregnancy test result after having two failed rounds of IVF with the first two of the three frozen embryos.
"I knew it was my last chance, so the fact it was a positive was so unbelievably wonderful," she said. "A truly wonderful, crazy life moment."
Bright's new baby is due in August, when she will be 45, and she said she had shared the news with co-star Lacey Turner early on as the actor who plays Stacey Slater was also pregnant at the same time.
Turner recently welcomed her second child, a son called Trilby Fox.
Watch: EastEnders' Lacey Turner has given birth to a baby boy